In the art under consideration a distinction is made between jet pulverizing systems or jet mills and whirl or vortex chamber mills. In one type of jet mill particles to be comminuted are introduced into the working fluid which is brought up to the high speed in a chamber owing to injecting thereof through one or more Venturi nozzles; moving in the high speed fluid flow, the particles collide with a target which may constitute reflective surfaces and/or other particles moving in different fluid flows in the chamber. In other words, in jet mills the particles are ground owing to the collision effect. Working speeds at which the particles of different materials move and get milled in the fluid flows in jet mills are substantially not less than 150-300 m/s. Such jet mills are described for example in U.S. Pat. No. 5,133,504. In another kind of jet mills the coarse particles are forced to collide with intersecting high speed fluid jets, thus obtaining even a higher resulting speed of interaction, and such technology is described for example in U.S. Pat. No. 4,546,926.
Neither of these kinds of jet mills is pertinent prior art with respect to the new technology being the subject of the present patent application.
It is also known to use whirl or vortex chambers in conjunction with jet mills for the classification of the ground material emerging from jet milling. In such combined system the relatively coarse particles are recirculated from the whirling classifier to the jet mill and such systems are described for example in U.S. Pat. Nos. 4,219,164, 4,189,102 and 4,664,319. It should be emphasized, that in such systems vortex chambers do not effect the milling operation, but rather the sorting.
Moreover, it has already been known to use whirl or vortex chambers for milling.
One modification of this technology is referred to, for example, in U.S. Pat. No. 4,502,641, and still constitutes a combination of the jet milling principle with a vortex chamber. A material to be comminuted is introduced into the vortex chamber through a venturi nozzle, i.e. at a speed of about 300 m/s. On the other hand, in the vortex chamber there is created a fluid flow vortex which rotates at a speed being much lower than the above mentioned value. During operation the particles injected into the chamber earlier become involved in the rotation of the relatively slow fluid vortex and thus turn to targets for the particles which continue to be injected through the venturi nozzle at the high speed. Such interaction results in collision between the particles in the vortex and the particles in the jet, i.e. ensures the comminution owing to the collision principle, as in the jet-mills mentioned above.
There are known milling vortex chambers which perform a so-called resonance whirl milling. Such a milling process differs from the jet milling process by a number of specific conditions, for example, by the speed of particles to be comminuted in the fluid flow, which in whirl chambers is considerably lower than that in jet mills. In these chambers there is no need in the high speed injection (through venturi nozzles) of the particles to be comminuted. Speed of the fluid flow in the nozzles of the vortex chamber is usually in the range of 50-130 m/s, and speed of the particles to be comminuted which move in the rotating fluid flow in the chamber is still lower and not greater than 50 m/s. It should be stressed that at such speeds jet mills turn to be totally useless. Owing to such specific conditions prevailing inside the whirl chamber the relatively coarse fed-in solid particles disintegrate spontaneously rather than in consequence of collision between the particles. It is generally believed that this effect is due to the fact that the coarse particles fed into the chamber, while rotating in the vortex, travel back and forth across the vortex thus passing a series of annular concentric zones with different values of fluid pressure so that in the course of their radial movement the particles are subjected to pressure gradients. In the course of a repeated back-and-forth motion a disbalance of pressure builds up in numerous cracks and cavities of the particles leading to gradual loosening of the particles' structure and eventually to spontaneous disintegration. Owing to this special milling principle the vortex chambers enable to comminute there inside such materials as rubber, paper, etc. i.e. the materials which can not be milled by colliding in jet mills. Moreover, super-hard abrasive materials, such as diamonds and boron nitride (BN), which cannot be milled by impact (collision), appeared to be comminutable in the resonance vortex chambers.
WO 94/08719 and SU 1,457,995 describe whirl chamber milling apparatuses fitted with tangential fluid injection nozzles and performing the so-called "resonance vortex grinding". The milling chamber comprises a generally cylindrical body with one or more openings serving for the introduction of a particulate solid matter to be comminuted. During the milling process, particles reaching dimensions substantially close to the required range of the milling are continuously discharged via an axial discharge duct. There may be further provided one or more sound generators placed each in the nozzle for interacting with the incoming fluid flow and thereby enhancing the grinding operation (WO 94/08719), or the chamber may be provided with a rotatable internal side wall adapted for rotation in the direction opposite to the vortex direction (SU 1,457,995).
It should be emphasized, that in each of the mentioned milling whirl chambers the comminution process, once initiated under specific parameters (such as the dimensions of the chamber, the volumetric flow rate and the viscosity of the working fluid, the size of the particles to be comminuted, etc.), will last until all the comminuted material is unloaded from the discharging passage of the chamber. In other words, between the moment the material is loaded to the milling whirl chamber and the moment it is completely discharged therefrom, the milling chamber acts as a black box.
None of the references known from the prior art deals with improving efficacy of the whirl milling chamber apparatuses, as such. More particularly, no means have been mentioned or described in the prior art for controlling the comminution process in the whirl chambers for deliberate adjusting the degree of comminution and uniformity of the milling which is expected to be obtained.